1. Field of the Invention
This invention relates in general to composite materials, and specifically to laminated composite materials with improved interlaminar strength.
2. Description of the Related Art
Laminated composite materials are widely used as construction materials in applications covering the range from printed circuit boards to aircraft skin. Typically, laminated composite materials are made by arranging a number of thin sheets of reinforcing material impregnated with an uncured binder in a stack, and then compressing the stack while the binder is cured by heat or other means. Each lamina of reinforcing material impregnated with an uncured binder is commonly called a "prepreg". The cured binder in the finished material is commonly referred to as the "matrix".
Laminated composite materials are susceptible to damage and delamination induced failures due to low interlaminar and intralaminar strength. Interlaminar and intralaminar damage can occur, for example, when runway debris impacts the skin of an aircraft during landing, or a tool is dropped on the aircraft during ground servicing. The mechanical properties of both the matrix and the reinforcing fiber control the interlaminar or intralaminar response mechanisms of a composite laminate, but it is primarily the mechanical properties at the interface between adjacent plies that control the interlaminar response.
Three methods are commonly used to improve the damage tolerance of composite materials: 1) adding a compliant layer between lamina in the laminate, 2) using a high failure strain matrix, and 3) inserting through-the-thickness (TTT) reinforcement in a laminate. It has yet to be conclusively proven which technique is best for improving the overall damage tolerance of laminated composite materials. It is likely that no single technique is universally optimal.
Compliant material between layers of composite material in a laminate allows relative movement between adjacent layers of composite material, and thereby reduces the interlaminar stresses along the interface of the compliant layer and the composite material. There are several techniques for adding a compliant layer between composite lamina. One technique is to place a layer of compliant film between composite lamina. Another technique is to add small compliant particles, such as small rubber spheres, between the layers of composite materials. Usually, the compliant film or compliant particles are applied to individual lamina during the fiber impregnation ("prepregging") process. These techniques are equally applicable to fabric, unidirectional tape and single yarn material forms. In general, the compliant material does not change the processing or handling qualities of the prepreg material. The compliant layer approach for improving damage tolerance is, however, not applicable to all composite material systems. For example, most organic matrix materials for elevated temperature applications are incompatible with the compliant layer techniques for toughening composite materials. Matrix materials for elevated temperature applications usually exhibit low failure strains which requires the designer to use a conservative design approach. The compliant layer also causes a reduction in the laminate fiber volume fraction, which proportionately reduces the in-plane mechanical properties of the composite laminate.
A high failure strain matrix, such as thermoplastic, also allows relative movement of adjacent layers. In this case the relative motion is due to the high strain capability of the matrix. High strain matrices and fibers also allow greater straining of a layer. Composite materials with compliant layers or composite materials of high failure strains are sometimes referred to as toughened composite materials. High strain matrix materials that have high stiffness are generally more expensive than high strain matrix materials that have lower stiffness. However, the high strain and high stiffness matrix materials are more difficult to process.
Through-the-thickness ("TTT") reinforcement increases the interlaminar strength and, to a limited extent, increases the intralaminar strength of the laminate thereby increasing the damage tolerance of a material. There are many types of through-the-thickness reinforcement and insertion methods. For example, stitching and tufting have been successfully applied to dry fiber preforms prior to resin infiltration and processing into composite laminates as a method of improving damage tolerance.
While stitching or tufting of dry fiber preforms improve damage tolerance, it has significant costs associated with it. Nominal stitch densities are between 60 and 160 per square inch. Using a single needle stitching machine requires between 1 and 3 minutes to stitch a one square inch region. When this process is applied to an entire wing skin or fuselage of an aircraft, it is obvious that considerable time and hence cost are involved in stitching. Furthermore, the preform can be damaged from the stitching/tufting process and from preform handling when it is transferred from the stitching/tufting tooling to the infiltration and cure tooling. The automated stitching/tufting machines, the infiltration equipment, and the development of resins compatible with the infiltration process are all costly. Most of the resins that can be infiltrated into dry fiber preforms have mechanical properties substantially inferior to those of the state-of-the-art toughened resins ordinarily used with prepreg materials. These inferior mechanical properties adversely affect the load carrying ability of the composite.
Through-the-thickness (TTT) reinforcements have also been applied to composite laminates composed of prepreg material. While stitching and tufting techniques have met with little success, the insertion of metal and composite rods as TTT reinforcement has been successfully applied to multilayer stacks of prepreg material.
One known method for insertion of rods into a stack of prepregs requires the forming of small diameter holes using a hollow ultrasonic needle, and then inserting a rod, either metallic or composite, through the hollow needle into the stack of prepreg. This process is repeated across the surface of the prepreg stack until the desired density of TTT reinforcement is obtained. The rods typically extend from the upper to the lower surface of the prepreg stack. Although this discontinuous reinforcement improves the damage tolerance of the composite material, currently this technique is labor intensive and economically impractical.
Another technique for inserting rods through the thickness of stacks of composite prepreg is by a foam block transfer method. Discontinuous rods are first inserted through the thickness of a layer of foam. After stacking the layers of prepreg, the rod filled foam block is placed on the surface of the prepreg stack. The assembly of prepreg laminate and foam block is bagged in the conventional manner and cured. As the temperature increases in the cure cycle, the foam softens and the viscosity of the resin decreases. When pressure is applied in the cure process the foam collapses and the rods are forced into the composite laminate. A substantial cost is incurred in making the rod filled foam layers, and the collapsed remains of a foam block may be unacceptable as a residue on a composite material. Further, the foam block transfer method is not readily applicable to cocured, built-up structures such as stiffened panels.